3D modeling of phytoplankton seasonal variation and nutrient budget in a southern Mediterranean Lagoon.

A 3D coupled physical-biogeochemical model is developed and applied to Bizerte Lagoon (Tunisia), in order to understand and quantitatively assess its hydrobiological functioning and nutrients budget. The biogeochemical module accounts for nitrogen and phosphorus and includes the water column and upper sediment layer. The simulations showed that water circulation and the seasonal patterns of nutrients, phytoplankton and dissolved oxygen were satisfactorily reproduced. Model results indicate that water circulation in the lagoon is driven mainly by tide and wind. Plankton primary production is co-limited by phosphorus and nitrogen, and is highest in the inner part of the lagoon, due to the combined effects of high water residence time and high nutrient inputs from the boundary. However, a sensitivity analysis highlights the importance of exchanges with the Mediterranean Sea in maintaining a high level of productivity. Intensive use of fertilizers in the catchment area has a significant effect on phytoplankton biomass increase.

Dynamic energy budget model: a monitoring tool for growth and reproduction performance of Mytilus galloprovincialis in Bizerte Lagoon (Southwestern Mediterranean Sea).

Mussel farming is the main economic activity in Bizerte Lagoon, with a production that fluctuates depending on environmental factors. In the present study, we apply a bioenergetic growth model to the mussel Mytilus galloprovincialis, based on dynamic energy budget (DEB) theory which describes energy flux variation through the different compartments of the mussel body. Thus, the present model simulates both mussel growth and sexual cycle steps according to food availability and water temperature and also the effect of climate change on mussel behavior and reproduction. The results point to good concordance between simulations and growth parameters (metric length and weight) for mussels in the lagoon. A heat wave scenario was also simulated using the DEB model, which highlighted mussel mortality periods during a period of high temperature.